Ground fault circuit interrupter (GFCI) devices are designed to trip in response to the detection of a ground fault condition at an AC load. Generally, the ground fault conditions results when a person comes into contact with the line side of the AC load and an earth ground at the same time, a situation which can result in serious injury. The GFCI device detects this condition by using a sensing transformer to detect an imbalance between the currents flowing in the line and neutral conductors of the AC supply, as will occur when some of the current on the line side is being diverted to ground. When such an imbalance is detected, a circuit breaker within the GFCI device is immediately tripped to an open condition, thereby opening both sides of the AC line and removing all power from the load.
GFCI devices have been provided in various forms, including portable or line cord devices and central units which provide protection for the AC wiring throughout a structure. More commonly, however, GFCI devices are incorporated into wall-mounted AC electrical receptacles which are designed for installation at various locations within a building. A typical receptacle configuration consists of a housing adapted to be received within a standard electrical box, with a pair of standard two- or three-prong AC outlets, a test pushbutton and a reset pushbutton accessible through the front of the housing. At the rear of the housing, two pairs of screw terminals are ordinarily provided. One pair of screw terminals allows the line and neutral sides of an AC source to be connected to the GFCI receptacle, and these terminals (referred to as the source or input terminals) are connected to the electrical outlets at the front of the housing via the GFCI circuitry and circuit breaker within the housing. The second pair of screw terminals, which are sometimes referred to as load or feedthrough terminals, are connected directly in parallel with the contacts of the AC outlets. This provides the installer with the option of connecting a standard, non-GFCI AC receptacle in parallel with the GFCI receptacle, in order to provide ground fault protection for the standard receptacle without the need to provide a separate GFCI circuit. The standard receptacle may be located remotely from the GFCI receptacle, but will ordinarily be close enough (e.g., in the same room) so that convenient resetting is possible when a ground fault condition occurs.
Unfortunately, there is a problem with GFCI receptacles of the type described above, in that the installer may erroneously connect the incoming AC source conductors to the load or feedthrough terminals of the receptacle rather than to the source or input terminals. Because of the nature of the internal wiring of the GFCI receptacle, this miswiring condition is not easily detected. AC power will still be present at the receptacle outlets, making it appear that the receptacle is operating normally. If the test pushbutton is depressed, the circuit breaker within the GFCI receptacle will be released and the reset pushbutton will pop out, again making it appear that the GFCI receptacle is operating normally and providing the desired ground fault protection. In reality, however, no such protection is being provided because the AC source has been wired directly to the receptacle outlets without passing through the internal circuit breaker of the GFCI device.
It is known to provide a GFCI receptacle with a visual indicator, such as a light-emitting diode (LED), to indicate that the circuit to which the receptacle is connected is operating normally. Such an arrangement is illustrated, for example, in U.S. Pat. Nos. 4,412,193, 4,568,997 and 5,202,662, all to Bienwald et al. If the receptacle has been wired properly, the LED is extinguished when a ground fault condition occurs. However, in the event that the receptacle has been miswired by connecting the AC source to the load terminals rather than to the input terminals, the LED is not extinguished when a ground fault condition occurs. Thus, when the test button is depressed to simulate a ground fault condition, the LED will remain illuminated to serve as an indication that the receptacle has been miswired. Unfortunately, however, the installer of the receptacle cannot necessarily be relied upon to understand the operation of the LED unless the installer is familiar with the type of GFCI receptacle being installed. That is, the failure of the LED to extinguish after the test button is depressed may not be interpreted by the installer as an abnormal condition, particularly since the installer will observe the popping out of the reset button that occurs during a successful test of a properly wired GFCI receptacle. Thus, the miswiring of the receptacle may not be detected and a dangerous condition may be allowed to persist.